The invention relates to a plastic pipe having a socket, the pipe comprising a body portion and at least at one of its ends the socket which has at least two layers.
The invention further relates to a method of providing a plastic pipe with a socket, in which method at least one end of a body portion of the plastic pipe is heated and expanded so as to form the socket.
The socket portion is typically the weakest portion of a pipe. To reinforce the socket, Finnish Patent 94902 teaches a solution for generating in the socket an orientation which differs from the orientation of the rest of the pipe. However, such a forcefully orientated material is not always suitable for the inner surface of a socket. In some plastics, for example, the solvent glue may trigger stress-cracking in the surface, thus making a glue joint impossible. Finnish Patent 945107, in turn, discloses a socket comprising an expanded pipe end and a sleeve adapted inside it. However, even such a solution does not often sufficiently improve the characteristics of the socket.
It is an object of the present invention to provide a plastic pipe and a method for avoiding at least some of the above drawbacks.
The plastic pipe of the invention is characterized in that the orientation of at least one layer of the socket differs from the orientation of another layer, and the orientation of at least one layer differs from the orientation of the body portion.
The method of the invention is characterized in that the plastic pipe comprises at least two layers and that the end of the plastic pipe is expanded so as to provide the plastic pipe with a socket, the orientation of at least one layer of which socket differs from the orientation of another layer and the orientation of at least one layer differs from the orientation of the body portion.
An essential idea of the invention is that the pipe comprises a socket having at least two layers and that the orientation of at least one layer differs from the orientation of the body portion of the pipe. A further essential idea is that the orientation of at least one layer of the socket differs from the orientation of another layer of the socket. The idea of a preferred embodiment is that the socket is composed of a portion of the pipe which is formed broader and of a sleeve disposed therein. The pipe is expanded for example at a temperature which does not substantially change its orientation, and the orientation of the socket differs from the orientation of the pipe, or the pipe is expanded at a temperature which changes its orientation, and the orientation of the socket differs from that of the expanded portion of the pipe. It is the idea of another preferred embodiment that the socket is composed of the end of a pipe having at least two layers, the end being expanded so that a socket forms therein. For example the layers of the pipe are of a different material so that their orientation temperatures are different. Before the end of the pipe is expanded, said end is heated to a temperature which causes orientation in the material of at least one layer during the expansion but which substantially causes no orientation in the material of at least one layer during expansion. Furthermore, a different orientation in different layers can be achieved for example by heating only one surface of the feedstock pipe. The surface being heated reaches such a high temperature that the tension brought about by the drawing relaxes immediately and the surface does not get oriented, but the inner, colder surface does get oriented.
A preferred construction is achieved when the material is oriented by drawing, but also contains staple fibres. Ideally, the socket comprises two layers which are reinforced by fibres and in which the opposite fibre directions are at an angle of about 50 degrees with respect to the axis and provide pressure resistance. The third layer is provided with an axial reinforcement which thus makes the orientation different as compared with the other layers. In a tension resistant pipe, said axial layer is needed only in the socket portion, not in the pipe. Finally, the entire construction is covered with a thermoplastic fibre-free inner and outer layer.
It is an advantage of the invention that the mechanical strength of the socket becomes as good as or even better than the rest of the pipe. By arranging different orientations to the socket layers, at least one layer, for example, is provided with an axial orientation which makes the socket extremely tension resistant, and at least one layer with a radial orientation which makes the socket pressure resistant. On the other hand, at least one layer may be oriented, whereby the mechanical and, possibly, also barrier characters, of the socket improve and at least one layer may be arranged substantially non-oriented, whereby its chemical characteristics are good.
The orientation of the pipe and/or socket can be achieved by different methods. Molecular orientation is concerned when it is achieved for example by drawing or shearing (for example a rotating mandrel in a tool). Different grades of orientation may also be achieved by making some layer of the pipe an/or the socket from a fibre-reinforced thermoplastic. The present invention only relates to self-reinforcing thermoplastic pipes and/or thermoplastic pipes reinforced with staple fibres. Plastic pipes reinforced with continuous fibres are not covered by the invention since the pipe cannot be provided with a socket by expansion. Instead, staple fibres glide in matrix plastic and present no problems for expanding the socket.
In the present description, the term xe2x80x98socketxe2x80x99 refers to that portion of the end of a pipe which is arranged broader than the rest of the pipe either by expanding the pipe or by arranging a sleeve on the outside of the pipe. Further, the term xe2x80x98a socket having at least two layersxe2x80x99 implies that at least one layer of the socket is an integral part of the wall of the rest of the pipe and the sleeve may constitute the second layer, or both the pipe and the socket may have at least two layers such that the socket layers are integral with the corresponding pipe layers. In the latter case thus refers to an integral pipe socket having at least two layers.